The present invention relates generally to the balancing of a rotatably mounted object used in motion control system. This invention is particularly applicable to a multi axis computer numerically controlled (CNC)machine in which a grinding wheel is moved to operate on a work piece mounted in a rotatable chunk or headstock, and it will be convenient to describe the invention in relation to the exemplary, but non-limiting, application.
The development of multi axes and multifunction machine tools in conjunction with the development of sophisticated computer controlled operations has facilitated the emergence of a generation of very high speed precision machine tools capable of complex multi-step operations from one machine. Such machine tools typically hold a work piece in a controlled position whilst a movable cutting tool acts upon the work piece to perform a desired task.
Typically a CNC machine is controlled by a computer program which serially instructs the machine to perform a sequential series of discrete operations in a predetermined sequence so that the movable operative part moves along a programmed path determined by the part program. Each individual instruction is termed a “block” and many constitute a determining command for each or a combination of controllable axes. For example, a block may instruct a chuck holding a work piece to move 5 mm in a Y axis at a given velocity. Multiple axis blocks may instruct the chuck to rotate and move forward 0.05 mm in the X and Y axis at a given velocity. Another block may instruct a grinding wheel to be rotated about a C-axis to a predefined angular position and then rotated about a spindle when the work piece is bought into contact with the grinding wheel. The blocks, once programmed into the computer, are then fixed in a set sequential order. The whole set of sequential blocks may then be automatically operated by the CNC machine which then operates from start to finish of the part program.
One of the objects subject to high speed rotation and movement in a CNC machine is the grinding wheel. Over time, the grinding wheel can develop an imbalance. The imbalance causes radial movement in the spindle to which the grinding wheel is mounted, resulting in the precision of the grinding being diminished and the grinding wheel being subject to uneven wear.
In order to attempt to correct the imbalance, it is possible for an operator to manually affix a weight to one of the planar surfaces of the grinding wheel at a location that counteracts the imbalance of the grinding wheel. However, it is an extremely difficult task for an operator to identify the size of a suitable weight and the location on the grinding wheel at which to place that weight. Using trial and error to attempt to obtain an optimal weight size and location can be lengthy and ultimately unsuccessful at correcting the imbalance.
The nature of the imbalance can also be assessed by the mounting of external accelerometers about the spindle on which the grinding wheel is mounted. However, the cost of this extraneous equipment and the difficulty of mounting the accelerometers in an industrial environment make this technique difficult to implement.
It would therefore be desirable to provide a manner of computing and/or correcting imbalance in a rotatably mounted grinding wheel or other object in a motion control system, such as a CNC machine, that enabled the imbalance to be accurately determined.
It would also be desirable to enable the imbalance to be accurately identified without requiring extraneous analytical equipment to be used.
It would moreover be desirable to provide a method of computing and/or correcting an imbalance in a rotatably mounted grinding wheel or other object in a motion control system, such as a CNC machine, that ameliorates or overcomes one or more problems of existing motion control systems.